The quick, accurate and reproducible measurement of pH and other ions is important to a number of industries. The ability to accurately measure pH provides important quality parameters in the medical field when it is used for the clinical analysis of blood and other body fluids. Likewise, pH is used as an important quality parameter for the control of milk when it arrives at the dairy, to indicate the freshness of meat, to measure the neutrality of treated industrial wastes and to monitor the acidity of rain.
The ability to measure pH is also an important process control parameter in a number of fermentation process. pH is used to measure the alkalinization of boiler feed water with NH.sub.3. pH is also used to monitor the precipitation of heavy metal ions in industrial waste water. In the field of chemistry, pH is used to control the adaptation of redox potentials and to optimize the dissociation of weak electrolytes. pH is also used to monitor the exhaustion of caustic gas scrubber solutions. Also in the field of chemistry, pH is used in the endpoint detection of potentiometric titrations, including in acidimetry, alkalimetry and in the titration of weak acids and bases in organic solvents.
Thus, various types of pH and other ion sensors have been developed to meet the above-referenced needs. In particular, ion sensors have been developed which include for example, chemical and electrochemical sensors. Chemical sensors are measuring devices which convert input variables of a system, such as for example, the concentration of certain species in air, water, solids, solvents, heterogeneous mixtures, or other systems of interest into analytically useful electrical signals.
Electrochemical sensors, however, are the largest and oldest group of sensors. These sensors are divided into three general categories based on what they measure: potentiometric sensors measure voltage; amperometric sensors measure current; and conductimetric sensors measure conductivity. Electrochemical pH sensors include (1) a sensitive electrode (SE) that measures the activity of protons and (2) a reference electrode (RE) whose potential must be constant. The SE and RE are in contact with the solution under investigation. In many cases, the accuracy of potentiometric pH measurements is determined by the reference electrode. Commonly used reference electrodes include silver/silver chloride and mercury/calomel electrodes.
Many problems exist with the use of the above-referenced electrochemical pH sensors. In particular, the reference electrodes are subject to potential drift, can operate only at very narrow temperature ranges, are subject to leakage and chemical reaction between the sample solution and the reference electrolyte and are highly toxic.
Thus, alternatives to electrochemical pH sensors have been developed. An example of such an alternative ion sensor is the sensing electrode. One example of such a sensor is the hydrogen electrode which is formed by a small wire or a small piece of platinized Pt foil over the surface of which pure hydrogen is bubbled. Such electrodes, however, are expensive, difficult to set up and to use. Thus, the hydrogen/platinum electrode nowadays is used only for thermodynarnic investigations or for the accurate determination of the pH values of non-reducible buffer solutions.
The glass pH electrode is another example of a sensing electrode. In recent years, the glass pH electrode has tended to supplant all other types of sensing electrodes for pH measurement [1-2]. For in-line pH sensing, although the glass-electrode is by far the most attractive choice of chemists, it is not suitable for certain applications, such as for example, clinical and food applications. In particular, glass electrodes suffer from the draw back that they are, for example, mechanically fragile, have high impedance and are susceptible to dehydration/alkali errors at low/high pH conditions.
Some of the problems of glass electrodes could be reduced by eliminating the internal reference buffer between the inner glass membrane surface and the Ag/AgCl electrode immersed in this buffer [3]. The problem consists in developing a stable contact that provides a reversible transition from the ionic to the electronic part of the sensor. In view of these problems, emphasis in recent years has shifted to the development of an all-solid-state sensor alternative for the glass electrodes.
Glass electrodes for sodium ion sensing first developed by Lengyed and Blum [4], have been studied systematically for interferences and other limitations by Eisenman et al. [5]. Modern ATI ORION sodium electrodes are glass electrodes with an internal system that eliminates temperature dependent drift. These electrodes are very stable and have relatively fast response times but also have a number of disadvantages: they are affected by fouling of glass, high temperatures, have a limited pH application range (7-11) and cannot be used for certain applications in, for example, the food industry [2].
Several tungsten oxide bronzes having the general formula A.sub.x M.sub.y WO.sub.3, where A is Na, K, Rb, Li, Co and Tl and M is K, Li and NH.sub.4 in polycrystalline form have been proposed for application as ion selective electrodes by Dobson et al. [6-7]. These investigations, as well as other studies have suggested the use tungsten and other bronzes as pH sensors [8-11 ] and the application of Na.sub.x WO.sub.3 for Na.sup.+ sensing [12].
Ternary molybdenum bronzes, for example, have been of interest in recent years due to their interesting physical properties, such as their highly anisotropic transport properties [13-14] and their superconductivity [15-16]. Such compositions have been reviewed by Hagenmuller [17] and in more detail recently by Greenblatt [18]. In contrast to the tungsten bronzes, the molybdenum bronzes (Mo-bronzes) are stoichiometric and more stable. Furthermore, good quality single crystals of the Mo-bronzes used as sensors have clear advantages compared to polycrystalline samples. Preliminary investigations by the inventors have indicated that certain molybdenum bronze single crystals can be used as pH sensors [19]. The inventors have also found that a sodium molybdenum bronze (Na.sub.0.9 Mo.sub.6 O.sub.17) electrode is sensitive to changes of hydrogen ion concentration, but also shows significant cross sensitivity to other ions, e.g. Li.sup.+, Na.sup.+, K.sup.+.
Accordingly, a need exists for metal oxide pH and sodium ion sensors that are operable over wide pH (3-9) and temperature (20.degree. C.-200.degree. C.) ranges. Furthermore, a need exists for metal oxide pH and sodium ion sensors which are stable, provide highly reproducible readings, have low cross-sensitivity to other ions and are dynamic over a wide range of operating conditions. In particular, a need exists for a pH and sodium ion sensor suitable for use in the food industry that has all of the advantages set forth above and without the disadvantages noted in the prior art. These and other objects which will appear from the following description are obtained with the compositions and devices according to the present invention.